1. Field of the Invention
This invention relates to a plasma display panel, and more particularly to a plasma display panel that is adaptive for improving contrast.
2. Description of the Related Art
Generally, a plasma display panel (PDP) radiates a fluorescent body using an ultraviolet with a wavelength of 147 nm generated upon discharge of an inactive mixture gas such as He+Xe or Ne+Xe, to thereby display a picture including characters and graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development. Particularly, since a three-electrode, alternating current (AC) surface-discharge PDP has wall charges accumulated in the surface thereof upon discharge and protects electrodes from a sputtering generated by the discharge, it has advantages of a low-voltage driving and a long life.
Referring to FIG. 1, a discharge cell of the conventional three-electrode, AC surface-discharge PDP includes a first electrode Y and a second electrode Z provided on an upper substrate 10, and an address electrode X provided on a lower substrate 18. The first electrode Y and the second electrode Z include transparent electrodes 12Y and 12Z, and metal bus electrodes 13Y and 13Z having a smaller line width than the transparent electrodes 12Y and 12Z and provided at one edge of the transparent electrodes 12Y and 12Z, respectively.
The transparent electrodes 12Y and 12Z are usually formed from indium-tin-oxide (ITO) on the upper substrate 10. The metal bus electrodes 13Y and 13Z are usually formed from a metal such as chrome (Cr) on the transparent electrodes 12Y and 12Z to thereby reduce a voltage drop caused by the transparent electrodes 12Y and 12Z having a high resistance. On the upper substrate 10 provided with the first electrode Y and the second electrode Z in parallel, an upper dielectric layer 14 and a protective film 16 are disposed. Wall charges generated upon plasma discharge are accumulated into the upper dielectric layer 14. The protective film 16 prevents a damage of the upper dielectric layer 14 caused by a sputtering during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made from magnesium oxide (MgO).
A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 provided with the address electrode X. The surfaces of the lower dielectric layer 22 and the barrier ribs 24 are coated with a fluorescent layer 26. The address electrode X is formed in a direction crossing the first electrode Y and the second electrode Z. The barrier rib 24 is formed in parallel to the address electrode 20X to prevent an ultraviolet ray and a visible light generated by a discharge from being leaked to the adjacent discharge cells. The fluorescent layer 26 is excited by an ultraviolet ray generated during the plasma discharge to generate any one of red, green and blue visible light rays. An inactive mixture gas is injected into a discharge space defined between the upper and lower substrate 10 and 18 and the barrier rib 24.
Such a PDP drives one frame, which is divided into various sub-fields having a different discharge frequency, so as to express gray levels of a picture. Each sub-field is again divided into an initialization period for initializing the entire field, an address period for selecting a scan line and selecting a cell from the selected scanning line and a sustain period for realizing the gray levels depending on the discharge frequency.
Herein, the initialization period is divided into a set-up interval supplied with a ramp-up waveform and a set-down interval supplied with a ramp-down waveform. For instance, when it is intended to display a picture of 256 gray levels, a frame interval equal to 1/60 second (i.e. 16.67 msec) is divided into 8 sub-fields SF1 to SF8 as shown in FIG. 2. Each of the 8 sub-fields SF1 to SF8 is divided into an initialization period, an address period and a sustain period as mentioned above. Herein, the initialization period and the address period of each sub-field are equal every sub-field, whereas the sustain period are increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field, thereby displaying a picture according to the gray levels.
FIG. 3 is a waveform diagram of a driving signal applied to the electrodes shown in FIG. 1.
Referring to FIG. 3, the PDP is divided into an initialization period for initializing the full field, an address period for selecting a cell, and a sustain period for sustaining a discharge of the selected cell for its driving.
In the initialization period, a ramp-up waveform rising slowly from a first voltage Vs lower than a discharge initiation voltage until a second voltage Vr going beyond the discharge initiation voltage is applied to all the first electrodes Y in the set-up interval. This ramp-up waveform causes a weak set-up discharge within cells of the entire field to generate wall charges within the cells.
The set-up discharge is divided into a surface discharge generated between the first electrode Y and the second electrode Z and an opposite discharge generated between the first electrode Y and the address electrode X. Herein, the surface discharge forms negative wall charges at the first electrode Y while forming positive wall charges at the second electrode Z. Further, the opposite discharge forms negative wall charges at the first electrode Y while forming positive wall charges at the address electrode X. Meanwhile, a majority of lights emitted at the surface discharge are progressed into an observer. This increases an emission amount of the lights in the initialization period that is a non-display period, and thus deteriorates a contrast characteristic to that extent.
In the set-down interval, after the ramp-up waveform was applied, a ramp-down waveform falling slowly at a first voltage Vs lower than a peak voltage (i.e., a second voltage Vr) of the ramp-up waveform is applied to the first electrodes Y. If the ramp-down waveform is applied to the first electrodes Y, then a weak erasure discharge occurs within the cells to thereby erase spurious electric charges of wall charges and space charges generated by the set-up discharge and uniformly leaves wall charges required for the address discharge within cells of the entire field.
In the address period, a negative scanning pulse Scan is sequentially applied to the first electrodes Y and, at the same time, a positive data pulse data is applied to the address electrodes X. A voltage difference between the scanning pulse Scan and the data pulse data is added to a wall voltage generated in the initialization period to thereby generate an address discharge within the cells supplied with the data pulse data. Wall charges are generated within the cells selected by the address discharge. Meanwhile, a positive direct current voltage having a sustain voltage level Vs is applied to the second electrodes Z during the set-down interval and the address period.
In the sustain period, a sustaining pulse sus is alternately applied to the first electrodes Y and the second electrodes Z. Then, a wall voltage within the cell selected by the address discharge is added to the sustain pulse sus to thereby generate a sustain discharge taking a shape of the surface discharge between the first electrode Y and the second electrode Z whenever the sustain pulse sus is applied. Finally, in the erasure period, an erasing ramp waveform erase having a small pulse width is applied to the second electrode Z to erase the sustain discharge.
Such a conventional PDP repeats the initialization period, the address period and the sustain period at all the sub-fields to thereby display a desired picture. However, the conventional PDP has a disadvantage in that contrast is deteriorated due to a light generated by the set-up discharge (particularly, surface discharge) in the initialization period. In other words, spurious lights is generated due to the set-up discharge that does not contribute to the brightness, and hence deteriorate the contrast of the PDP.
For instance, a full white of the PDP driven with five sub-fields has a brightness of approximately 154 cd/m2. At this time, a light generated by the reset discharge has a brightness of approximately 0.75 cd/m2. Thus, the conventional PDP driven with five sub-fields has a low contrast ratio of approximately 1:205. Similarly, the conventional PDP driven with ten sub-fields has a low contrast ratio of approximately 1:300.